dhclient on virtual interface in Xubuntu

dhclient on virtual interface in Xubuntu

I configured a vlan interface on an xubuntu box and set it up for dhcp
eth1.125  Link encap:Ethernet  HWaddr d4:ae:52:6a:dd:81
      inet addr:10.10.10.4  Bcast:10.10.10.255  Mask:255.255.255.0
      inet6 addr: fe80::d6ae:52ff:fe6a:dd81/64 Scope:Link
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:167 errors:0 dropped:0 overruns:0 frame:0
      TX packets:82 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:0
      RX bytes:7620 (7.6 KB)  TX bytes:11719 (11.7 KB)
my /etc/network/interfaces file contains the following
auto eth1.125
iface eth1.125 inet dhcp
The dhcp did work and I got an IP. However, I want to release/renew the IP on this interface. If I do "dhclient -r eth1.125", it releases the IP but I cant do "dhclient eth1.125" to renew. I have to reboot the box to get back an IP in the virtual interface. How can I renew without reboot ?